Fix an exception on wx-3.0-gtk3 in PLCOpenEditor when generating ST files.

On Python2.7 with WX3.0 and GTK3, an assertionError rises on generating a ST file if the name is already set. The first generation works without problems, if you generate the file a second one, PLCOpenEditor tries to open the file
save dialog with the pre entered name from last run. Then the following assertion pops up:

PyAssertionError: C++ assertion "volDummy.empty() && pathDummy.empty()" failed at ./src/common/filename.cpp(568) in Assign(): the file name shouldn't contain the path

This fix reduces the filepath of the ST file the to the filename. Now it works fine.

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#Encoding of BACnet Object Types;;;

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#Code;Object Type;;

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0;Analog Input;;

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1;Analog Output;;

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2;Analog Value;;

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3;Binary Input ;;

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4;Binary Output;;

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5;Binary Value;;

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6;Calendar;;

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7;Command;;

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8;Device;;

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9;Event-Enrollment;;

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10;File;;

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11;Group;;

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12;Loop;;

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13;Multistate Input;;

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14;Multistate Output

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15;Notification Class

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16;Program

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17;Schedule

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18;Averaging

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19;Multistate Value

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20;Trend Log

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21;Life Safety Point

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22;Life Safety Zone

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23;Accumulator

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24;Pulse Converter